The vacuum cassette in present use is a sealed container with a door providing access to the semiconductor wafers. When the cassette is in its normal operating mode with a vacuum within, the door cannot be opened to remove or load wafers. In the case of interfacing the vacuum cassette with a vacuum semiconductor processing system (vacuum processing system), the cassette is loaded into a vacuum load-lock interface integral with the process machine. This load lock is sealed and pumped down to a vacuum approximately the same as is inside the cassette, and the cassette door is then opened.
A gate valve in the load-lock is then opened and the wafers moved to and from the vacuum cassette to the interior of the vacuum processor as required. The wafers remain in a vacuum throughout the processing steps. When the wafer movement is complete, the gate valve and the cassette door are closed, and the load-lock is backfilled to ambient pressure with clean gas, usually air. The load-lock door can then be opened and the vacuum cassette removed. The interior of the vacuum processor remains in a vacuum throughout the process.
The above process is done automatically after the introduction of the vacuum cassette into the load-lock chamber. Vacuum gages are installed in appropriate locations to monitor the load-lock and process chamber conditions as required by a process sequencer/controller.
If it is desired to interface the vacuum cassette to a process that is not done in a vacuum, or if there is a circumstance where the semiconductor wafers must be at room ambient pressure, the cassette must be placed inside a vacuum load-lock chamber, which is then evacuated to a pressure comparable to the pressure inside the cassette. The cassette door may then be opened and the chamber and cassette backfilled with an appropriate atmosphere, usually clean air, to ambient pressure. Then the load-lock chamber may be opened, either with a gate valve or the loading door, and the wafers removed from or loaded into the open cassette.